The Medicare Prescription Drug, Improvement and Modernization Act of 2003 established a new Medicare drug benefit (Part D) that gives beneficiaries access to a private Medicare prescription drug plan. The new law had particular relevance for the 6.4 million dual eligibles, low-income elderly and individuals with disabilities who are enrolled in both Medicaid and Medicare since it resulted in a shift in their prescription coverage from Medicaid to private Part D plans. As a result of this transition, many dual eligibles had increases in their co-payments and faced differential (or tiered) co-payments for the first time under Part D. The impact of these changes may be particularly acute for dual eligible patients with schizophrenia. Antipsychotic drugs are the mainstay therapy for this mental illness. Compliance with these drugs is critical to reduce acute psychotic episodes and psychiatric hospitalizations. This project s three aims: (1) To determine the impact of the transition to Medicare Part D, both (i) overall and (ii) by level of state assistance provided on antipsychotic medication use and non-drug mental health care service use; (2) To determine the impact of changes in prescription co-payments under Part D, namely (i) increases in minimum co-payments and (ii) increases in generic/brand co-payment differentials on antipsychotic medication use and non-drug mental health care service use; and (3) To determine the net impact of Medicare Part D and prescription co-payment changes on total (drug + non-drug) mental health care costs in dual eligible patients with schizophrenia. The study will conduct a multi-state analysis using a quasi-experimental design by linking pre-Part D data on dual eligible patients with schizophrenia across all state Medicaid programs with their Medicare claims and Part D files. Our study findings will help guide design and refine federal- and state-level policies surrounding Part D so as to provide a balance between medication access and costs in such vulnerable patients who can least afford to be without needed medications. PUBLIC HEALTH RELEVANCE: Our study findings will inform policymakers about the unintended negative and positive consequences of the transition to Medicare Part D in dual eligible patients with schizophrenia. They will help guide design and refine policies surrounding Part D so as to provide a balance between medication access and costs in such vulnerable patients who can least afford to be without needed medications.